Direct current permanent magnet electric motors are widely used in many applications. Recently there has been a trend towards increased usage of such motors in order to meet automotive engine cooling system requirements. Such motors are advantageous in that, for normal cruising speeds, the flow of air through an automotive radiator is sufficient, without the fan motor being operated, for adequate cooling of the engine. However, occasional traffic situations require minimal operation of the cooling fan at less than full speed, while extremely hot days or high heat load (such as when the air conditioner is in operation) full speed fan operation is necessary to provide sufficient engine cooling. Thus, in the automotive environment, multiple speed fans are desirable.
A multiple speed automotive fan is shown in U. S. Pat. No. 5,113,104 entitled "Structured Product Dynamoelectric Machine" issued to Blaettner et al. on May 12, 1992, said patent herein incorporated by reference.
Permanent magnet DC motors utilize two or more brushes contacting a commutator which provides the direct current flow to the windings of the rotor, which in turn provide the desired magnetic repulsion/attraction with the permanent magnets located around the periphery of the motor. The brushes are conventionally located in brush boxes and utilize a U-shaped spring which biases the brush into contact with the commutator. Because such springs generally contact the brush at only two points (at the rear of the brush), the stability of the brush and accuracy of its positioning with respect to the commutator is adversely affected. The brushes can move from side to side in the brush box resulting in movement of the brush relative to the commutator and bouncing of the brush so as to interrupt contact with one or more commutator segments during operation of the motor. This resulting interruption in current flow to the motor can result in arcing at the brush/commutator interface and overheating of the brush, as well as decreased current flow to the motor and a resultant decrease in motor output and increase in motor noise.
Prior art fan motors, especially those utilized in the automotive environment, have a difficult task in keeping the fan motor itself cool while performing its engine cooling task. It has been found that the brush/commutator interface is the second most significant source of generated heat in a DC brush motor. Without effective cooling in this area, there are higher thermally induced resistance losses and decreased brush life due to higher operating temperatures along with thermal aging effects. If brushes are left exposed to the environment, in order to promote cooling of the brushes, they can become contaminated by dirt, water, oil and other contaminants present in the automotive environment. It is desirable to cool the brushes without exposing them to contaminants.
It is desirable to be able to control the output speed of automotive DC brush motors (full speed and less than full speed). This is normally accomplished by the use of external means i.e., electronic controllers or power resistors or internal means i.e., separate windings and commutators and extra brushes. Where electronic controllers are utilized, complex electronic circuitry is necessary to provide the desired voltage and current which will result in the desired output speed change of the motor necessitating increased cost of the overall component.
If internal or external power resistors are used, such resistors are included in the motor energization circuit with the result that the resistor dissipates a substantial portion of the power (in the form of heat) which would otherwise be applied to the motor thus wasting battery energy and increasing motor temperature. Where internal means, such as extra brushes, windings and commutators are used, a number of components and elements in a motor are increased generally by a factor of 2, where a separate operating speed is necessary, again complicating the manufacturing process and raising the ultimate cost of the motor.
Of concern to automotive component designers and, in particular, automotive DC motor designers, is the compatibility of their products with existing automotive wiring systems. It is desirable to be able to obtain variations in fan operating speed by applying a fixed system voltage (battery voltage) to one terminal for high speed operation and another terminal for low speed operation, rather than having to incorporate a switch or electronic relay or a variable voltage supply system to obtain multi-speed operation.
Furthermore, there are situations in which the motors are prevented from rotation i.e., a stalled condition (this can result from snow packing of the fan, minor crash damage, etc.). If the motor is energized during the stalled condition, the relatively high current load, without sufficient cooling air, can overheat the wiring resulting in a fire in the engine compartment. While most direct current electric motors in automotive applications can be protected by means of fuses or of thermoswitches, such protection is not feasible for under-the-hood applications because of the extremely wide temperature range required in this environment.
Moreover, the operation of a permanent magnet direct current motor creates radio frequency interference (RFI) as a result of the connection and disconnection of electric current with the rotor windings in the brush/commutator interface. While this cannot be eliminated, it is desirable to reduce RFI as much as possible, especially in automotive applications.